There is known in the art an abrasive process in which an axial vibratory tool is brought into pressure contact with a workpiece surface in the presence of abrasive grits. The grits may be a part of the tool by being secured thereto, or otherwise supplied in a fluid suspension into the polishing interface. The axial tool is longitudinally vibrated at a frequency typically in an ultrasonic range to produce highly accelerating forces which drive the abrasive grits to impact upon the workpiece surface and cause an accelerated abrading action thereover. This technique has hitherto been employed primarily for shaping purposes where a substantial amount of stock is to be removed roughly from blank materials such as carbides, ceramics and glass which are commonly brittle.
When an attempt is made to apply the abrasive process described to the precision-finishing of a shaped workpiece with an intricate surface contour, as encountered in the die or mold making field, on an automatic basis, there arise difficulties as to the accuracy and finish quality of the processed surface as well as the rate of finishing attainable by the process. Firstly, the surface finish obtained thereby is relatively poor. Secondly, it would be necessary to displace the active portion of a tool relative to the workpiece precisely along a given path corresponding to the shaped contour in the workpiece while holding a polishing tool and workpiece interface interposed by the abrasive grits.
To this end, numerical path control or template copying control may be employed to advance the tool along a programmed path and at a rate of advance necessary to maintain the required polishing tool and workpiece interface interposed by the abrasive grits. In establishing the rate of advance, inasmuch as the roughly shaped contour for finishing is characterized by a surface irregularity and hence the slight amount of stock to be finish-removed varies from one position to another on the contour it is necessary to set the rate at a minimum value in consideration of a portion requiring the maximum amount of stock removal. Thus, the operation is unavoidably time-consuming.
Furthermore, the tool wear, insofar as unavoidable, must be taken into account in determining the actual path of the tool advance. Without determination of the precise information of the tool wear, finishing would never be achieved with a due precision because a tool shift from a given contour cannot be obtained so as to compensate for the tool wear. In the process described, however, the tool wears unavoidably as biased or with unevenness over its peripheral surface, and yet in a highly complicated format which it is indeed impossible to determine insofar as the precise details of the surface irregularity of the shaped workpiece surface and hence the precise amounts of localized stock removal as required are practically undefinable which are further complicated when taken with the precise details of intricacy of the shaped contour to be finished. Thus, if a path of the tool advance is adequately defined according to the finishable surface contour of a shaped workpiece, it would be almost impossible to provide a properly modified path for the actual advance of the tool relative to the workpiece which precisely compensates for the tool wear.
Moreover, while it may be desirable to maintain a constant axial or lateral pressure to urge the tool against the finishable surface, the abrading action would unavoidably change due to the large or irregular change in the effective abrading contact area so that the accuracy and finish quality of the polished surface contour would be far less than satisfactory.